1. Field of the Invention
The present invention relates to a method and device for generating or creating interrogation channels or modes in an IFF (Identification Friend or Foe) type system using spread-spectrum waveforms.
The invention can be applied especially in IFF identification systems used in military aviation working in Mode 5. Mode 5 is an encrypted interrogation/response mode using a waveform defined in part V of the NATO standardization agreement, STANAG 4193, and its own enciphering algorithm. FIG. 2 recalls the structure of this waveform.
Hereinafter in the description, the term “channel” designates an encrypted interrogation/response mode implemented in the IFF question-and-answer identification systems using Mode 5 but using different enciphering algorithms.
2. Description of the Prior Art
At present, there are no means of differentiating between the messages when several enciphering algorithms are used simultaneously or again when different enciphering keys are used simultaneously for one and the same enciphering algorithm.
An interrogator chooses to interrogate with a given enciphering algorithm A or with specific keys (Algo A). A responder does not know the algorithm or the key with which the messages that it receives are enciphered There are then various possibilities:                if the responder is equipped only with the algorithm A, it will decipher the interrogation message and respond.        If the responder is equipped only with the algorithm B (Algo B) which is different from the algorithm A, it will not be able to respond to the interrogations but its cryptographic computer will be used to try and decipher the message with the algorithm A.        when the responder is equipped with the algorithms A and B, it will have to process the messages with both algorithms, either in parallel or serially, if the processing times permit it. It will respond with the algorithm that will have given a comprehensible and valid interrogation message.        
The idea of the present invention consists especially in creating independent channels by which it is possible to have, simultaneously, several types of enciphering algorithms or several different keys for one and the same enciphering algorithm and by which the system can process the messages without resorting to parallel or serial processing operations. The system has the possibility of recognizing the different messages whatever the enciphering algorithm or the enciphering keys used and of determining the algorithm to be used to decrypt the interrogations and the responses.
To this end, the symbols of the interrogation message header are modulated and the responder, by detecting the modulation of the header, knows which algorithm or which key has been used.
The interrogations made with an algorithm A do not disturb a responder that is not equipped with the algorithm A. In fact, the two interrogation/response modes, namely the algorithm A and the algorithm B, implemented in the interrogator-responder system ignore each other and do not disturb each other. This actually prevents additional processing operations and prevents the computer from being kept unnecessarily busy.